1. Field of the Invention
The present invention relates to an optical element driving apparatus. In particular, though not exclusively, the present invention can be applied to an optical element used in an exposure apparatus.
2. Description of the Related Art
As an optical element driving apparatus for driving an optical element, Japanese Patent Laid-Open No. 2003-203860 (corresponding to U.S. Pat. No. 6,788,386) discusses a technique of positioning the optical element with six degrees of freedom by an actuator. Japanese Patent Laid-Open No. 2003-203860 also discusses that the optical element driving apparatus further includes a sensor and an actuator to constitute a control loop for the purpose of correcting a local mirror surface defect.
Japanese Patent Laid-Open No. 2004-64076 (corresponding to U.S. Pat. No. 6,842,277) discusses a technique of performing position adjustment of a deformable mirror through servo control of a force actuator for driving the deformable mirror and a position sensor for measuring a contact point of the force actuator with the deformable mirror.
FIG. 14 is a block diagram of a control system 40 in an optical element driving apparatus discussed in Japanese Patent Laid-Open No. 2004-64076. A control command given to an actuator 30 is decided by a controller 45 to make the position of the deformable mirror, which is determined from a measured value of the position sensor 42, matched with a predetermined position target value. In accordance with the control command, the mirror is driven so that the mirror position determined from the measured value of the position sensor is matched with the predetermined position target value.
In the optical element like the above-mentioned deformable mirror, there is a possibility that, depending on the arrangement of measurement points of the position sensors each of which measures the position of the optical element, the measured value of the position sensor may include both a rigidity displacement and an amount of deformation of the optical element, which are caused by the influence of deformation of the optical element. In such a case, the optical element cannot be controlled to an accurate position even when the position of the optical element is controlled using the measured values of the position sensors.